1. Field of the Invention
The invention relates to electronic components, and particularly to a balun.
2. Description of Related Art
A balun is a device for transforming signals between a balanced architecture and an unbalanced architecture. The signal of the balanced structure includes two balanced signals with a phase difference of 180 degrees. The signal of the unbalanced architecture includes an unbalanced signal. Thus, the balun can transform the unbalanced signal to the two balanced signals with a 180-degree phase shift and vice versa, i.e., two balanced signals to an unbalanced signal.
The balun is often used in wireless local area networks (WLANs) and mobile communication devices. Due to a desire to make smaller mobile communication devices, the size of the balun needs to be reduced. Recently, transmission line resonators and capacitors are widely used to reduce the size of a transmission line balun. A conventional transmission line balun includes a one-quarter (¼) wavelength balun and a one-sixteenth ( 1/16) wavelength balun.
FIG. 1 is a schematic diagram of a conventional 1/16 wavelength balun. The conventional balun, disposed on a substrate 50′, includes a first transforming part 10 and a second transforming part 20. The first transforming part 10 is symmetrical with respect to a central line thereof. The first transforming part 10 includes a first coupled line 12, a first transmission line 14, a second transmission line 16, and a micro capacitor C1′. A center of the first coupled line 12 includes a ground via. The first transmission line 14, the first coupled line 12, and the second transmission line 16 are connected in series. The first transmission line 14 is bent, and includes a first line node 140 and a balanced port 2′. The second transmission line 16 is bent, and includes a line node 160 and a balanced port 3′. The first line node 140 is connected to the second line node 160 via the micro capacitor c1′.
The second transforming part 20 includes a second coupled line 22, a third transmission line 24, a fourth transmission line 26, a second capacitor C2′, and a third capacitor C3′. The third transmission line 24, the second coupled line 22, and the fourth transmission line 26 are connected in series. The third transmission line 24 is bent, and includes a third line node 240 and an unbalanced port 1′. The fourth transmission line 26 is bent, and includes a fourth line node 260. The third line node 240 is connected to the fourth line node 260 via the micro capacitor C2′. The fourth line node 260 is grounded via the micro capacitor C3′. The micro capacitor C2′ and the micro capacitor C3′ are connected to an end 262 of the fourth line node 260 respectively in two vertical directions.
Lengths of the first coupled line 12 and the second coupled line 22 are 1/16 of a working wavelength of the conventional balun. The size of the conventional balun is relatively large because the micro capacitors C2′ and C3′ are connected to the end 262 respectively in two vertical directions. In addition, due to the ground via of the first coupled line 12, a length between the first line node 140 and the first coupled line 12 is long. Therefore, the size of the balun cannot be further minimized using a typical layout and structure.